An oven of this type is known from the document DE No. 1,236,105, which describes a door-closing safety device for a microwave oven. For this purpose a metal strip is connected to the oven door. When the door is not closed correctly light from a light source mounted on the oven frame is incident on this strip. This strip has a small aperture through which the light can reach a photo-electric cell when the door is closed correctly, thus enabling the magnetron to be energized. However, this device is restricted to a particular location and provides only a limited protection. During use the closing properties of the door will gradually deteriorate and microwave energy will be able to escape although the door may appear to be properly closed when the device in accordance with said document is used.
Likewise, this safety device does not make provision for the situation in which, although the device is operating correctly, there are nevertheless microwave energy leaks owing to the warping of the door. This is a highly critical problem with microwave ovens, since the safety standards in force require leaks to be very small around the entire door opening. Moreover, it is desirable for these standards to be met throughout the useful life of the oven. During use, the door may tend to become distorted or shift position, the standards are no longer met and users may become unacceptably irradiated.